As a theory, the planetary model was elegant, much more so than the ‘plum pudding’ version. But was it correct? To test his theory, Rutherford suspended a large magnet from the ceiling of his laboratory. Directly underneath, on a table, he fixed another magnet. When the pendulum magnet was swung over the table at a 45-degree angle and when the magnets were matched in polarity, the swinging magnet bounced through 90 degrees just as the alpha particles did when they hit the gold foil. His theory had passed the first test, and atomic physics had now become nuclear physics.13
For many people, particle physics has been the greatest intellectual adventure of the century. But in some respects there have been two sides to it. One side is exemplified by Rutherford, who was brilliantly adept at thinking up often very simple experiments to prove or disprove the latest advance in theory. The other project has been theoretical physics, which involved the imaginative use of already existing information to be reorganised so as to advance knowledge. Of course, experimental physics and theoretical physics are intimately related; sooner or later, theories have to be tested. Nonetheless, within the discipline of physics overall, theoretical physics is recognised as an activity in its own right, and for many perfectly respectable physicists theoretical work is all they do. Often the experimental verification of theories in physics cannot be tested for years, because the technology to do so doesn’t exist.
The most famous theoretical physicist in history, indeed one of the most famous figures of the century, was developing his theories at more or less the same time that Rutherford was conducting his experiments. Albert Einstein arrived on the intellectual stage with a bang. Of all the scientific journals in the world, the single most sought-after collector’s item by far is the Annalen der Physik, volume XVII, for 1905, for in that year Einstein published not one but three papers in the journal, causing 1905 to be dubbed the annus mirabilis of science. These three papers were: the first experimental verification of Max Planck’s quantum theory; Einstein’s examination of Brownian motion, which proved the existence of molecules; and the special theory of relativity with its famous equation, E=mc2.
Einstein was born in Ulm, between Stuttgart and Munich, on 14 March 1879, in the valley of the Danube near the slopes that lead to the Swabian Alps. Hermann, his father, was an electrical engineer. Though the birth was straightforward, Einstein’s mother Pauline received a shock when she first saw her son: his head was large and so oddly shaped, she was convinced he was deformed.14 In fact there was nothing wrong with the infant, though he did have an unusually large head. According to family legend, Einstein was not especially happy at elementary school, nor was he particularly clever.15 He later said that he was slow in learning to talk because he was ‘waiting’ until he could deliver fully formed sentences. In fact, the family legend was exaggerated. Research into Einstein’s early life shows that at school he always came top, or next to top, in both mathematics and Latin. But he did find enjoyment in his own company and developed a particular fascination with his building blocks. When he was five, his father gave him a compass. This so excited him, he said, that he ‘trembled and grew cold.’16
Though Einstein was not an only child, he was fairly solitary by nature and independent, a trait that was encouraged by his parents’ habit of encouraging self-reliance in their children at a very early age. Albert, for instance, was only three or four when he was given the responsibility of running errands, alone in the busy streets of Munich.17 The Einsteins encouraged their children to develop their own reading, and while studying math at school, Albert was discovering Kant and Darwin for himself at home – very advanced for a child.18 This did, however, help transform him from being a quiet child into a much more ‘difficult’ and rebellious adolescent. His character was only part of the problem here. He hated the autocratic approach used in his school, as he hated the autocratic side of Germany in general. This showed itself politically, in Germany as in Vienna, in a crude nationalism and a vicious anti-Semitism. Uncomfortable in such a psychological climate, Einstein argued incessantly with his fellow pupils and teachers, to the point where he was expelled, though he was thinking of leaving anyway. Aged sixteen he moved with his parents to Milan, attended university in Zurich at nineteen, though later he found a job as a patent officer in Bern. And so, half educated and half-in and half-out of academic life, he began in 1901 to publish scientific papers. His first, on the nature of liquid surfaces, was, in the words of one expert, ‘just plain wrong.’ More papers followed in 1903 and 1904. They were interesting but still lacked something – Einstein did not, after all, have access to the latest scientific literature and either repeated or misunderstood other people’s work. However, one of his specialities was statistical techniques, which stood him in good stead later on. More important, the fact that he was out of the mainstream of science may have helped his originality, which flourished unexpectedly in 1905. One says unexpectedly, so far as Einstein was concerned, but in fact, at the end of the nineteenth century many other mathematicians and physicists – Ludwig Boltzmann, Ernst Mach, and Jules-Henri Poincaré among them – were inclining towards something similar. Relativity, when it came, both was and was not a total surprise.19
Einstein’s three great papers of that marvellous year were published in March, on quantum theory, in May, on Brownian motion, and in June, on the special theory of relativity. Quantum physics, as we have seen, was itself new, the brainchild of the German physicist Max Planck. Planck argued that light is a form of electromagnetic radiation, made up of small packets or bundles – what he called quanta. Though his original paper caused little stir when it was read to the Berlin Physics Society in December 1900, other scientists soon realised that Planck must be right: his idea explained so much, including the observation that the chemical world is made up of discrete units – the elements. Discrete elements implied fundamental units of matter that were themselves discrete. Einstein paid Planck the compliment of thinking through other implications of his theory, and came to agree that light really does exist in discrete units – photons. One of the reasons why scientists other than Einstein had difficulty accepting this idea of quanta was that for years experiments had shown that light possesses the qualities of a wave. In the first of his papers Einstein, showing early the openness of mind for which physics would become celebrated as the decades passed, therefore made the hitherto unthinkable suggestion that light was both, a wave at some times and a particle at others. This idea took some time to be accepted, or even understood, except among physicists, who realised that Einstein’s insight fitted the available facts. In time the wave-particle duality, as it became known, formed the basis of quantum mechanics in the 1920s. (If you are confused by this, and have difficulty visualising something that is both a particle and a wave, you are in good company. We are dealing here with qualities that are essentially mathematical, and all visual analogies will be inadequate. Niels Bohr, arguably one of the century’s top two physicists, said that anyone who wasn’t made ‘dizzy’ by the very idea of what later physicists called ‘quantum weirdness’ had lost the plot.)